With the development of tumor radiology and material science, radiotherapy has gradually entered the age of “three precisions” including precise positioning, precise planning and precise therapy as an important means for treating cancers. In the prior art, a light limiting barrel has the advantages of small penumbra and low influence on healthy sites. However, the radiation field of the light limiting barrel is generally round, so that the shape of a focus cannot be fitted well; besides, the shape and size of the light limiting barrel cannot be adjusted in real time. What's more, different-sized light limiting barrels need to be customized for different patients, so that the light limiting barrel is inconvenient to use and very troublesome to replace. In order to improve the working efficiency, in a latest solution of the prior art, a storage box is arranged beside an accelerator, a plurality of different-sized light limiting barrels are put into the storage box. When a light limiting barrel needs to be replaced for another size, the light limiting barrel below the accelerator should be taken down and put into the corresponding position of the storage box for storage and later use, and then, the light limiting barrel having the required size is taken out and mounted to the working position. The solution standardizes field management of the light limiting barrels and also improves the efficiency by taking and putting nearby, but the improvement on efficiency is still limited, in particular, when a light limiting barrel and a grating blade system are used simultaneously, the light limiting barrel is often mounted between the accelerator and the grating, and when the light limiting barrel needs to be replaced, the grating system is detached first and then a new grating is replaced and mounted, so that the operation is very tedious and there is also potential risk of equipment damage.